The present invention relates to a direction finding system in which two primary sonic energy beams having respectively different frequencies generate a third beam at a frequency equal to the difference between the first two beam frequencies.
Such a system, which operates according to the so-called parametric principle, has the advantage that, with a relatively small transmitting area, it achieves a high transmission directivity with small ancillary lobes. The low difference frequency resulting from the frequencies of the two primary beams experiences smaller attenuation in the propagation direction so that good ranges can be realized. Moreover, the lower difference frequency beam can penetrate certain media better than the higher frequency primary beams.
In a known method of this type which is utilized, for example, to scan the bottom of a harbor, and is described in U.S. Pat. No. 3,824,531, the direction of the third sonic beam at the lower difference frequency resulting from the two primary beams is varied in that the directivity pattern of the first and second beams are electrically pivoted in a known manner and the echo signals which are received with no directional receiving characteristic by the receiving transducers are pre-equalized with respect to their direction.
Although this method is well suited for surveying a harbor where the water is of shallow depth, it is not suitable for surveying, for example, the bottom of the ocean in deep sea regions where depths up to 5000 m are not uncommon.
When scanning the bottom of the ocean with the prior art method, an ocean depth of 5000 m would require a period of time of about 6.6 seconds between the transmission of the sounding beam and the receipt of the bottom echo, this time period being determined by the depth of the ocean and the speed of sound in water. The scanning direction can thus be changed only about every 7 seconds.
The time required to scan a strip of bottom transversely to the direction of travel of the surveyor ship in, for example, 24 angularly offset directions would thus be 168 seconds. With a traveling speed of 4 knots for the surveyor ship, the ship will already have changed its position by about 340 m during that time. If thus, for mapping purposes, the bottom of the ocean is to be accurately surveyed with the prior art method, this requires an extremely slow travel of the surveyor ship; this makes the surveying task slow and also expensive.